When Pope Benedict XVI announced that he would abdicate the papacy, he explained that “in today’s world, subject to so many rapid changes…both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me.” By becoming the first pope to resign since the 15th century, Benedict demonstrated a self-knowledge that is incredibly rare among leaders. Contrast his behavior, for example, with that of New Jersey Senator Frank Lautenberg, who before being forced to stand aside described Newark Mayor Cory Booker as a “disrespectful” child for challenging his reelection bid, even though Lautenberg would have been almost 92 when he was sworn in.

It may be a fraught subject, but aging often has enormous effects on people’s personalities and cognitive function. Some leaders can maintain their vitality and abilities into extreme old age, but after enough time in office, a leader’s performance probably will decline, perhaps precipitously. And, although many scholars argue that leaders have little impact on foreign policy because political systems tend to produce dispensable candidates, there are specific circumstances in which individuals become enormously important — one of the most notable being when they change radically once in office, surprising the system.This is precisely what happens to anyone who spends a long time in senior government positions, because of both the effects of power itself on those who wield it, and the effects of age on every human being.

Power itself has profound, and usually toxic, effects on those who have it. CEOs are so pampered that comparing them to babies is surprisingly illuminating (and very funny). What is true for a CEO is, in this case, even more true for the men and women who lead nations and can literally have power over life and death. Over time this authority is likely to have profound effects on most people’s personalities. It would be remarkable indeed for any person treated with deference and pampering for years, even decades, to not be affected by it. Even worse, power tends to make those who have it more sociopathic. They become more impulsive, more Machiavellian, and more willing to dehumanize those who lack it. What’s more, leaders are almost invariably surrounded by family and staff who depend on them for continued access to the perquisites of power, and so often hide evidence of erratic behavior or decline. Woodrow Wilson’s wife Edith hid his crippling stroke, Nixon’s senior staff conspired to conceal his alcoholism, and Anthony Eden’s doctor helped cover up his illness and addiction to amphetamines during the Suez Crisis.

The effect of age is equally worrying. Aging can have a powerful and largely negative impact on leaders in three ways. It can greatly increase their vulnerability to illness, shift their personality, and decrease their cognitive abilities.

It is a sad fact of life that the passage of time depletes the energy of every person and renders all of us more vulnerable to illness. Physical ailments can have surprisingly powerful effects on decision-making. As Roy Baumeister and Jon Tierney describe in their book, willpower is depleted by conditions as seemingly minor as the common cold, making it considerably more difficult to delay gratification or make difficult decisions, because the cold depletes the blood glucose critical for brain function. Driving when you have a severe cold, for example, is statistically more dangerous than driving while mildly intoxicated. More broadly, in Presidential Leadership, Illness, and Decision Making, Rose McDermott described how illness can make leaders unpredictable, limit their attention spans, shorten their time horizons, and diminish their cognitive capacities. Wilson’s stroke, for example, intensified his natural rigidity and eliminated any last hopes of American entry into the League of Nations. In The Impact of Illness on World Leaders, Bert Park, a neurosurgeon, makes a powerful case that age-related dementia in Paul von Hindenburg was a key factor enabling Hitler’s rise to power. Hindenburg was 82 when he defeated Hitler to win re-election to the presidency of the Weimar Republic in 1932. He twice rejected any role for Hitler in the government, until, at the age of 84, his increasing weakness led to his tragic agreement in January 1933 to make Hitler chancellor of a government otherwise staffed by non-Nazis.

Even beyond the immediate effects of illness, aging can have pronounced effects on personality. Put simply, in general people really don’t mellow with age. Instead, Jerrold Post and Bert Park have shown that they tend to become exaggerated versions — almost caricatures — of themselves, with their normal tendencies and patterns becoming intensified. This tendency is particularly likely to affect foreign policy. The aggressive can become belligerent, the passive, apathetic. Tendencies that would otherwise have fallen within an acceptable range can suddenly become problematic — a shift that, when it happens to a head of government, is particularly likely to upset foreign policy.

Finally, and perhaps most troubling, are aging’s effects on cognition. Some of these are well known. The advance of age tends to weaken recall, particularly of recent events, for example. Less commonly acknowledged, but perhaps more important, are aging’s effects on intelligence. Cognitive abilities can be split into two categories: crystallized and fluid. Crystallized intelligence is what we use to accomplish routine tasks. It increases over the course of a person’s life, peaking in the 60s. Fluid intelligence, on the other hand, is the ability to solve new problems. It seems to begin declining at 20. This asymmetric deterioration is perhaps the most worrying feature of aging. The increase in crystallized intelligence can serve to camouflage any real decline that might be occurring. Most situations, after all, are routine, and so a leader may seem entirely unaffected by age. Furthermore, governments are likely to have considerable institutional ability to handle such situations, which will tend to compensate for a leader’s compromised skills.

The most critical and dangerous situations, on the other hand, are novel ones — situations that the normal functioning of governmental institutions is least able to handle and that therefore require peak performance from a leader. This is precisely when an age-related decline in fluid intelligence is likely to have its most severe effects. So age-related decline may be most consequential at the worst possible moment.

Given the potential dangers, the burden of proof should be on aging leaders to justify their continued hold on power, not on those who challenge them. It is certainly possible — even likely — that this presumption will sometimes force out leaders who could still make a valuable contribution. Remember, however, that most leaders have relatively little impact on events. Most leaders are far more dispensable than is generally believed — and certainly far more dispensable than they are likely to believe! Just as important, most leaders who do have an impact do so through poor performance, not brilliance. There are just many, many more ways to be a fool than there are to be a genius. The potentially foregone gains from removing an effective leader too early are far, far smaller than the harms that will be avoided by removing ineffective ones.

In the United States, this suggests the need for term limits for all senior officials who cannot easily be removed from office. Term limits have already been imposed on the presidency, of course, but they should be extended to include the Supreme Court, governors, and likely also the speaker of the House. When the Constitution was written, the life expectancy in the United States was under 30, so there was no need for any such requirement. The advance of medical technology, however, has made term limits overwhelmingly necessary. Of people aged 71 to 79 — an age to which very few people survived two centuries ago — 21 percent are likely to suffer from either cognitive impairment or dementia severe enough that it’s medically diagnosable. The odds that they will simply not be able to perform as well as they could have a decade or two earlier are likely far higher. Given the stakes of the decisions made by, for example, presidents and members of the Supreme Court, a 1 in 5 chance that the person making it is suffering from age-related cognitive decline is simply far too large to accept.

GE, the company perhaps more associated with good leaders and managers than any other, recognizes this danger and has a mandatory retirement age of 65 that applies even to the CEO. When the legendary Jack Welch hit the age cap, he was able to negotiate an extra year, but that was it. GE wisely realized that even he was replaceable and had Jeff Immelt waiting in the wings. The United States government would do well to learn, at least in this case, from the example of one of America’s most iconic companies. Pope Benedict had the humility and self-awareness to realize that he had reached the limit of his physical capabilities. We can, perhaps, expect that sort of wisdom from a religious leader, but it seems like far too much to expect political leaders to willingly follow his example. Right now, though, relying on their willingness to follow his example and recognize their own limitations is all we have. It is not nearly enough.